As is known, the fuel injectors for internal-combustion engines which are commercially available at present comprise: a main tubular body, which is provided with a central through pipe, which ends at an axial end of the tubular body, in a spray nozzle, which can project outside the injector a jet of finely sprayed fuel; a shutter unit, which is fitted such as to be axially mobile inside the central pipe, from and towards the position of closure, in which it closes the spray nozzle, such as to prevent discharge of the fuel; and a contrast spring which can maintain the shutter unit in the said position of closure.
Outside the main tubular body, the fuel injectors additionally comprise a coil made of electrically conductive material, which, when an electric current is passed through it, can generate a magnetic field which can overcome the resilient force of the spring, such as to move the shutter unit away temporarily from the position of closure, so as to permit controlled discharge of the fuel.
The shutter unit is obviously made at least partially of ferro-magnetic material, and in some types of injector, it consists of a cup-type body with a cylindrical shape, which is fitted such as to be mobile axially inside the central pipe, with its own base facing towards the spray nozzle, such that in the position of closure, it is disposed with its base abutting a flat surface provided on the spray nozzle.
Owing to the strong mechanical stresses to which it is subjected, the shutter unit, i.e. the cup-type body with a cylindrical shape, normally consists of a cylindrical tubular element made of ferro-magnetic material, and of a highly-resistant sealing disc made of rigid material, which is welded to an axial end of the said cylindrical tubular element. This disc has a considerable thickness, and is provided on its outer flat surface with two concentric annular projections which have the same height, and can provide the hydraulic sealing when they abut the flat surface which delimits the spray nozzle. Finally, in order to permit passage of the fuel, the sealing disc is provided with a plurality of through holes, which can put the interior of the cup-type body into communication with the outer flat surface of the disc, at the area delimited by the two concentric circular projections.
The main disadvantage of the above-described fuel injectors is that in the final part of their vital cycle, they are subject at intervals to undesirable blow-by of fuel, with a substantial increase in the pollutant emissions, which does not enable the internal-combustion engine to comply with the directives concerning pollutant emission.
In fact, in the long term, the repeated impacts of the shutter unit on the flat surface which delimits the spray nozzle give rise to localised resilient deformations, which in some cases do not allow the shutter disc, or rather its two concentric annular projections, to be placed so as to abut correctly the flat surface which delimits the spray nozzle, so as to provide the hydraulic sealing, and thus prevent discharge of the fuel from the spray nozzle.